1. Field of the Invention
This invention relates to single crystal nickel-based superalloys and, more particularly, single crystal nickel-based superalloys and articles made therefrom for use in advanced gas turbine engines under high stress, high temperature conditions.
2. Description of the Prior Art
Advances over recent years in the metal temperature and stress capability of single crystal articles have been the result of the continuing development of single crystal superalloys, as well as improvements in casting processes and engine application technology. These single crystal superalloy articles include rotating and stationary turbine blades and vanes found in the hot sections of gas turbine engines. However, gas turbine engine design goals have remained the same during the past decades. These goals include the desire to increase engine operating temperature, rotational speed, thrust-to-weight ratio, fuel efficiency, and engine component durability and reliability.
The basic technology of alloys for the casting of single crystal components is described in U.S. Pat. Nos. 3,494,709; 4,116,723 and 4,209,348. Development work resulted in first generation nickel-based superalloys, which were materially improved over those described in the aforementioned patents. However, these first generation nickel-based superalloys contained no rhenium. Examples of such first generation nickel-based superalloys, commercially known as CMSX-2 alloy and CMSX-3 alloy produced by Cannon-Muskegon Corporation, assignee of the present application, are described in U.S. Pat. No. 4,582,548. Further development work resulted in second generation nickel-based superalloys having improved creep strength/creep rate. These second generation nickel-based superalloys have a moderate rhenium content of about 3 weight percent. An example of such a second generation nickel-based superalloy is described in U.S. Pat. No. 4,643,782. This patent discloses a superalloy, commercially known as CMSX-4 alloy, having a specific nickel-based composition including a rhenium content in the range of 2.8-3.2 weight percent. The present invention provides the next generation of nickel-based superalloys having higher total refractory element (W+Re+Mo+Ta) content and improved mechanical properties.
Single crystal articles are generally produced having the low-modulus (001) crystallographic orientation parallel to the component dendritic growth pattern or blade stacking fault axis. Face-centered cubic (FCC) superalloy single crystals grown in the (001) direction provide extremely good thermal fatigue resistance relative to conventionally cast articles. Since these single crystal articles have no grain boundaries, alloy design without grain boundary strengtheners, such as carbon, boron and zirconium, is possible. As these elements are alloy melting point depressants, their reduction from an alloy design provides a greater potential for high temperature mechanical strength achievement since more complete gamma prime solution and microstructural homogenization can be achieved relative to directionally solidified (DS) columnar grain and conventionally cast materials. Their reduction also makes possible a higher incipient melting temperature.
These process benefits are not necessarily realized unless a multi-faceted alloy design approach is undertaken. Alloys must be designed to avoid tendency for casting defect formation such as freckles, slivers, spurious grains and recrystallization. Additionally, the alloys must provide an adequate heat treatment window (numeric difference between an alloy's gamma prime solvus and incipient melting point) to allow for nearly complete gamma prime solutioning. At the same time, the alloy compositional balance should be designed to provide an adequate blend of engineering properties necessary for operation in gas turbine engines. Selected properties generally considered important by gas turbine engine designers include: elevated temperature creep-rupture strength, thermo-mechanical fatigue resistance, impact resistance plus hot corrosion and oxidation resistance.
An alloy designer can attempt to improve one or two of these design properties by adjusting the compositional balance of known superalloys. However, it is extremely difficult to improve more than one or two of the design properties without significantly or even severely compromising the remaining properties. The unique superalloy of the present invention provides an excellent blend of the properties necessary for use in producing single crystal articles for operation in gas turbine engine hot sections.